1. Field of the Invention
The present invention relates to a message division communication method and a message division communication system for conducting transmission and receiving of messages in a communication system including a host, an intermediate device connected to the host through a high-speed bus, and a remote device connected to the host or the intermediate device through a low-speed bus.
2. Description of the Related Art
A communication system such as a switching system has a configuration for controlling a channel device or the like device by a host such as a central control unit for transmitting and receiving various messages between the host and the channel device to control the communication. In this communication system, a remote device may be connected through a low-speed bus. The transmission and receiving of messages to and from the remote device are also desirably carried out efficiently.
FIG. 1 is a schematic diagram for explaining the conventional communication system of this type. Reference numeral 61 designates a host such as a central control unit, numeral 62 an intermediate device such as a channel device, numeral 63 a remote device such as a remote line concentrator having a function equivalent to the channel device, numeral 64 a high-speed bus for executing a processor access control (PAC) communication protocol, and numeral 65 a low-speed bus for executing a LAPD (link access procedure on the D channel) or No. 7 protocol communication. The host 61 such as a central control unit and the intermediate device,62 such as a channel device are installed in the same office building, for example, and connected by a high-speed bus 64 such as a processor access control bus having a transmission rate of 20 Mbps, for example. Control commands and various messages such as status information are thus transmitted at high speed. The remote device 63 such as a remote line concentrator, on the other hand, is installed at a place remote from the host 61, and connected by a low-speed bus 65 such as LAPD or NO. 7 system bus having a transmission rate of 64 kbps, for example, for transmitting control commands and various messages such as status information.
In a signal error monitor of NO. 7 scheme, for example, the link is disconnected when a pulse train of all xe2x80x9c1sxe2x80x9d continues for 128 ms, for example. The maximum message length, therefore, is 273 bytes. Also, the maximum length is specified as 260 bytes for LAPD. In this way, the maximum message length for each protocol is determined to maintain the balance between the error monitor and the transmission rate. On the other hand, the message between the host 61 and the device 62 connected by a high-speed bus can be lengthened. The message length is shortened, however, between the host 61 and the remote device 63 which are connected by a low-speed bus through the intermediate device 62.
The device 62 such as a channel device connected to the host 61 such as a central control unit through the high-speed bus 64 has a function equivalent to that of the remote device 63 such as a remote line concentrator. Although the host 61 is connected to these devices of the same function, the maximum message length for transmission between the host 61 and the device 62 is different from that for transmission between the host 61 and the remote device 63 since they are connected to the host 61 through buses of different transmission rates.
Even in the case where a message as long as 4096 bytes can be transmitted through a high-speed bus of 20 Mbps, for example, the message length is limited to about 256 bytes when a low-speed bus of 64 kbps is used.
As described above, the host 61 is connected to the device 62 and the remote device 63 having the equivalent function through buses of different transmission rates. When transferring a large amount of messages such as the system setting information, therefore, different communication control schemes are required of the two different transmission routes. The resulting problem is a high cost required for program development and an increased processing burden imposed on the host 61.
The present invention has been developed in order to obviate the above-mentioned disadvantage of the conventional communication system, and the object thereof is to provide a novel communication method and communication system capable of transmitting and receiving messages by a common communication control scheme in a communication system including a host and a plurality of devices connected to the host through buses of different transmission rates.
According to a first aspect of the invention, there is provided a message division communication method used for transmitting and receiving messages in a communication system including a host, an intermediate device connected to the host through a high-speed bus and a remote device connected to the host through a low-speed bus, comprising the steps of the host designating and notifying the remote device of a maximum message length corresponding to the transmission rate of the low-speed bus, dividing the message into division messages each not exceeding the designated maximum message length, and transmitting and receiving the division messages between the host and the remove device.
As a result, the transmission of messages using a common communication scheme becomes possible in a communication system having both high-speed and low-speed buses therein.
Also, the division messages transmitted are reassembled into the original message in the host or the remote device.
Further, the maximum message length is designated from the host to the remote device according to the transmission rate of the low-speed bus by way of a device connected to the host through the high-speed bus.
In this way, an efficient message transmission is made possible even in a communication system having high-speed and low-speed buses in coexistence.
According to a second aspect of the invention, there is provided a communication system comprising a host, an intermediate device connected to the host through a high-speed bus, a remote device connected to the intermediate device or the host through a low-speed bus, wherein the host includes a communication control unit for notifying the remote device of the maximum message length determined in a way corresponding to the transmission rate of the low-speed bus and transmitting the division messages of the message to the remote device in a length not exceeding the maximum message length, and wherein the remote device includes a communication control unit for receiving and reassembling division messages from the host thereby to receive the mess age from the host and send a message to the host in division messages each not exceeding the maximum message length designated by the host.
Thus, a message is transmitted without being divided between devices connected by a high-speed bus, and can be transmitted by being divided between devices connected by a low-speed bus. As a result, a message can be transmitted and received efficiently using a common communication control scheme in a communication system comprising devices of different communication rates.